How to place a vehicle on a sculpted landscape?
In my driving games, I use a python script that's supplied with the Blender docs for controlling my vehicles. The link to that script (and example) is here. This script makes everything a lot easier and offers greater control of your vehicle than with logic bricks. 

In studying the script, you'll see that the body of the vehicle is a rigid body with Collision Bounds turned on, and it's suspended on the wheels by constraints. The wheels are set to No Collision with Collision Bounds turned on. This keeps the vehicle body off the ground plane while still letting the wheels rest on it without that jumpiness, which is normally caused by the collision between rigid bodies and static objects.